From German Patent DE 198 36 370 C2, it is known to produce the tapering portion of the foundation screw, which portion is embodied conically, by in-mold hammering. The conical portion can comprise a plurality of conical subportions which have a different conicity. In a first embodiment, the anchoring portion is embodied in one piece with the retaining portion, and alternatively, the two portions are embodied as separate components and are joined together for instance by suitable joining technology, resulting in a kink in the outer contour of the foundation screw in the transition region.
From German Patent Disclosure DE 10 2008 043709 A1, a method for producing a rotary foundation anchor is known, in which the tapering region of the rotary foundation anchor is formed by compression of the free end as a result of moving at least two molded bodies toward one another until they are together. In the production of the tapering region, which is a conical region, the result is a definite kink. Because of the production process, this kink has at best a radius in the range of the wall thickness of the tube that is used as a blank.
In such abrupt changes in the outer contour of foundation screws, it has proved disadvantageous that they present increased resistance to being screwed in as the foundation screw is being introduced into the ground. Moreover, the possibility exists that the soil in the vicinity of the cylindrical retaining portion will lift up, thus reducing the stability of the screwed-in foundation screw in the radial and axial directions.
From German Utility Model DE 93 16 438 U1, a foundation screw is moreover known which has a tubular basic body with an encompassing helical screw thread for being screwed into the ground. The basic body has a substantially cylindrical first longitudinal portion, a tapering second longitudinal portion, and a third longitudinal portion. The first longitudinal portion merges tangentially with the third longitudinal portion via one convex and one concave jacket region of the second longitudinal portion. The convex jacket region has at least one radius of convexity R1 which has at least the value of the tube diameter D of the first longitudinal portion.